Tar sand, which also can be referred to as bituminous sand or oil sand, includes aggregates of sand, clay and bitumen. Most tar sand also contains water. Tar sand deposits can be found in many parts of the world, including North America. One of the largest tar sand deposits is in the Athabasca region of Alberta, Canada. It is estimated that the Athabasca tar sand deposit contains the equivalent of about 250 billion barrels of oil. By way of comparison, the proven worldwide oil reserve is estimated to be about 850 billion barrels.
Extracting usable oil from tar sand typically begins by separating the bitumen from the mineral components. With conventional technology, the high cost of this process makes oil derived from tar sand more expensive than oil from other sources. More readily available sources of oil, however, are depleting rapidly. Thus, there is an increasing need for methods capable of economically separating large quantities of bitumen from tar sand.
In addition to their high cost, conventional methods for obtaining bitumen from tar sand typically have an adverse impact on the environment. For example, many of these methods require significant quantities of finite resources, such as water and natural gas. Many conventional methods also form undesirable byproducts, such as fine tailings. Fine tailings typically are made up of clay, sand, water, and residual bitumen. Due to the water entrained in the clays, fine tailings can have a sludge-like consistency that lasts indefinitely. Byproducts such as fine tailings can be stored in ponds, but these ponds are costly to build and maintain and can be damaging to the local environment, including the local water supply.
Many conventional methods for obtaining bitumen from tar sand also have serious technical limitations. For example, many conventional methods use water, which can cause clays in the tar sand to swell and interfere with processing equipment. In addition, some conventional methods result in the undesirable precipitation of soluble asphaltenes.
One example of a conventional method is described in U.S. Pat. No. 4,046,668 (the '668 patent). The '668 patent discloses the extraction of hydrocarbons from tar sands with a mixture of light naphtha having from 5 to 9 carbon atoms per molecule and methanol. The method disclosed in the '668 patent is limited, in part, because it requires the simultaneous use of two solvents, which increases processing costs.
U.S. Pat. No. 4,347,118 (the '118 patent) discloses a method in which pentane is used to extract bitumen from tar sands. The method disclosed in the '118 patent requires the use of two fluidized bed drying zones. Operation of these fluidized bed drying zones requires a large amount of energy, thereby limiting the efficiency of the overall method.
U.S. Pat. No. 5,143,598 (the '598 patent) discloses a method that includes adding heptane to tar sand to form a bitumen-rich heptane phase and then displacing the bitumen-rich heptane phase with water. This method is limited, in part, because it requires steam vaporization and condensation, which are low-efficiency processes. Also, the use heptane, a non-aromatic solvent, can result in the precipitation of the heptane insoluble asphaltene fraction present in the bitumen phase. In addition, using water generates large amounts of aqueous waste. Use of water also can introduce undesirable impurities into the bitumen, such as chlorine, and can result in undesirable swelling of clays in the tar sand. Furthermore, the bitumen recovered by this method typically has a low purity and requires additional processing, such as by centrifugation. This further increases the cost of the overall method.